Data acquisition systems and methods may be used in a variety of applications. For example, data acquisition techniques may be used in nuclear magnetic resonance imaging systems and Fourier transform spectrometer systems. Such techniques also may be used in mass spectrometer systems, which may be configured to determine the concentrations of various molecules in a sample. A mass spectrometer operates by ionizing electrically neutral molecules in the sample and directing the ionized molecules toward an ion detector. In response to applied electric and magnetic fields, the ionized molecules become spatially separated along the flight path to the ion detector in accordance with their mass-to-charge ratios.
Mass spectrometers may employ a variety of techniques to distinguish ions based on their mass-to-charge ratios. For example, magnetic sector mass spectrometers separate ions of equal energy based on their momentum changes in a magnetic field. Quadrupole mass spectrometers separate ions based on their paths in a high frequency electromagnetic field. Ion cyclotrons and ion trap mass spectrometers distinguish ions based on the frequencies of their resonant motions or stabilities of their paths in alternating voltage fields. Time-of-flight (or “TOF”) mass spectrometers discriminate ions based on the velocities of ions of equal energy as they travel over a fixed distance to a detector.
In a time-of-flight mass spectrometer, neutral molecules of a sample are ionized, and a packet (or bundle) of ions is synchronously extracted with a short voltage pulse. The ions within the ion source extraction are accelerated to a constant energy and then are directed along a field-free region of the spectrometer. As the ions drift down the field-free region, they separate from one another based on their respective velocities. In response to each ion packet received, the detector produces a data signal (or transient) from which the quantities and mass-to-charge ratios of ions contained in the ion packet may be determined. In particular, the times of flight between extraction and detection may be used to determine the mass-to-charge ratios of the detected ions, and the magnitudes of the peaks in each transient may be used to determine the number of ions of each mass-to-charge in the transient.
A data acquisition system (e.g., an integrating transient recorder) may be used to capture information about each ion source extraction. In one such system, successive transients are sampled and the samples are summed to produce a summation, which may be transformed directly into an ion intensity versus mass-to-charge ratio plot, which is commonly referred to as a spectrum. Typically, ion packets travel through a time-of-flight spectrometer in a short time (e.g., 100 microseconds) and ten thousand or more spectra may be summed to achieve a spectrum with a desired signal-to-noise ratio and a desired dynamic range. Consequently, desirable time-of-flight mass spectrometer systems include data acquisition systems that operate at a high processing frequency and have a high dynamic range.
In one data acquisition method, which has been used in high-speed digital-to-analog converters, data is accumulated in two or more parallel processing channels (or paths) to achieve a high processing frequency (e.g., greater than 100 MHz). In accordance with this method, successive samples of a waveform (or transient) are directed sequentially to each of a set of two or more processing channels. The operating frequency of the components of each processing channel may be reduced from the sampling frequency by a factor of N, where N is the number of processing channels. The processing results may be stored or combined into a sequential data stream at the original sampling rate.